CINCINNATI, Ohio -- It now appears clear that the major mass extinctions in Earth's history were caused by sudden, catastrophic global events. However, University of Cincinnati geologist Arnie Miller now has evidence that radiations, or rapid bursts of diversification, have much more regional or local origins.
Miller will present his data at Monday, Oct. 20 during the annual meeting of the Geological Society of America in Salt Lake City. Professor Miller has compiled an extensive data base of Ordovician fossils, including information about where they were found, when they first appeared in the fossil record and the type of environment in which they were found.
When he compared rates of diversification at the local level with reported global diversification rates, he found significant differences in the patterns. "The global pattern does not match any localized pattern," said Miller. "The global diversity trend is misleading for what it says about the rates of transition and perhaps even for what causes them."
Miller looked at patterns for each of the major land masses or paleo-continents existing during the Ordovician. The global pattern is fairly smooth, showing a steady increase in diversity through the first half of the Ordovician. In contrast, diversification showed a more abrupt or spiky pattern at the local level.
Diversification also peaked at different times on different paleo-continents. One key factor appear to be tectonic activity and the emergence of volcanoes and mountain ranges. When this occurs, sediments erode into the ocean providing a plentiful supply of new substrates and nutrients for marine organisms. "That may have turned on the spigot for diversification in a big way," said Miller.
However, Miller's analysis does not support a competing theory for diversification in the Ordovician either. That theory used "ecological evolutionary units" to depict rapid or "knife-edge" changes in the major fauna worldwide. Miller argues that the "knife-edge" transitions did occur on a local level. They simply didn't occur at the same time in localities worldwide.
"There appear to be windows of opportunity associated with diversification," said Miller. "and maybe those transitions are far more abrupt than we appreciate. But those transitions, for whatever reasons, took place at different times in different places."
Miller's research is supported by NASA's Program in Exobiology.